1. Field of the Invention
The present invention relates generally to epitaxial growth systems for production of semiconductor materials and devices, in particular. More specifically, the invention relates to the design of hydride vapor phase epitaxy (HVPE) growth systems and reactors, the design of internal components of HVPE growth systems and reactors, and HVPE-based processes for growth of group III-nitride materials and devices that can be used in optoelectronics as well as in high-power high-frequency electronics.
2. Prior Art
This application references a number of patents, applications and/or publications. Each of these patents, applications and/or publications is incorporated by reference herein.
The usefulness of gallium nitride (GaN), aluminum nitride, indium nitride, and their ternary and quaternary compounds (AlGaN, InGaN, AlInGaN), collectively known as “group III-nitrides,” has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices (see T. Nishida and N. Kobayashi, Phys. Stat. Sol. (a), 188 (1), 113 (2001); S. Nakamura, G. Fasol, and S. J. Pearton, The Blue Laser Diode. New York: Springer, 2000; and L. F. Eastman and U. K. Mishra, IEEE Spectrum, 39 (5), 28 (2002)). These devices are typically grown epitaxially by growth techniques including molecular beam epitaxy (MBE) (see S. Yoshida, S. Misawa and S. Gonda, Appl. Phys. Lett. 42 (1983), pp. 427), metalorganic chemical vapor deposition (MOCVD) (see H. M. Manasevit, F. M. Erdmann and W. I. Simpson, J. Electrochem. Soc. 118 (1971), pp. 1864), or hydride vapor phase epitaxy (HVPE) (see H. P. Maruska and J. J. Tietjin, Appl. Phys. Lett. 15 (1969), pp. 327). Among these three techniques, HVPE has the advantage of a high growth rate, which is more than a factor of ten higher than those inherent to MOCVD or MBE, making HVPE most preferable for the growth of thick III-Nitride films, templates, free-standing substrates, and bulk crystals.
During these deposition processes, a group III-nitride is grown upon a substrate or template consisting of, but not limited to, sapphire, silicon, silicon carbide, magnesium aluminate spinel, gallium nitride, aluminum nitride, aluminum-gallium nitride alloys, indium nitride, and/or lithium aluminate. A template shall be understood to be a substrate of one of the preceding materials coated with a layer of group III-nitride material. For the purposes of this invention, the terms “substrate” and “template” will be used interchangeably, though one skilled in the art will recognize that slightly different growth chemistries are required to optimize a group III-nitride deposition process for each. The differences in required chemistries are independent, however, of the implementation of the invention as described below.
Group III-nitride-based optoelectronic and electronic device performance to a great extent depends on the structural perfection of the starting template or substrate material. Due to the lack of widespread availability and high cost of native substrates (a substrate consisting of the same group III-nitride as the group III-nitride device structure), it is common for group III-nitride-based devices to be grown upon templates. These templates most commonly consist of group III-nitride layers on sapphire, silicon or silicon carbide substrates. Most commonly, the templates for visible optoelectronic devices consist of GaN thin films on sapphire or silicon substrates. As such, while this disclosure will refer principally to “GaN templates,” herein, such references incorporate all templates that incorporate group III-nitride films regardless of composition. As the structural quality of the template increases, the performance of the device grown on it tends to improve. It is well known that quality of many orientations of group III-nitride templates can be improved by increasing thickness of the GaN epilayer. In thick epilayers defect density can be reduced down to the level well suited even for the highly delicate device structures.
Since hydride vapor phase epitaxy provides the highest possible rate for the GaN growth, HVPE has emerged as a primary technique for the GaN template production. High growth rates in the vapor phase can be achieved at high operating gas flows that continuously resupply group III and group V precursor molecules to the surface of the growing film. However, high gas flow rates require that special attention be paid to the gas flow distribution to provide high quality homogeneous growth.
US Patent Application Publication No. 2010/0215854 discloses a method and apparatus that may be utilized in HVPE deposition processes. This application discloses two passages/channels that introduce the metal and nitrogen containing precursor gases into the HVPE growth chamber. These passages may be separated spatially in an effort to prevent mixing of the metal-containing and nitrogen-containing precursor gases until they reach a substrate. An inert gas may also be flowed down through the passages to help maintain separation and limit reaction at or near the passages, thereby preventing unwanted deposition on the passages. Embodiments described in the patent application describe a showerhead design of the reactor for use in HVPE deposition. As it was found in S. A. Safvi, N. R. Perkins, M. N. Horton, A. Thon, D. Zhi, T. F. Kuech, Optimization of reactor geometry and growth conditions for GaN halide vapor phase epitaxy, Mat. Res. Soc. Symp. Proc. 423 (1996) 227-232, the main drawback of the showerhead design is the tight coupling between the deposition temperature and gas flow rate condition. In experiments in which the substrate holder was kept close to the inlet, the resulting epilayers have a dark polycrystalline patch in the center of the wafer, with a clear single crystalline film at the edges. It was noticed that the polycrystalline patch decreased in size and eventually disappeared as the substrate was moved further away from the inlet.
In the paper devoted to the modeling of the HVPE showerhead reactor (S. A. Safvi, N. R. Perkins, M. N. Horton, A. Thon, D. Zhi, T. F. Kuech, Optimization of reactor geometry and growth conditions for GaN halide vapor phase epitaxy, Mat. Res. Soc. Symp. Proc. 423 (1996) 227-232), the importance of raw materials species transport has been demonstrated. In particular, the computations have revealed an essential role of natural concentration convection in the species transport, resulting in large recirculation zones, vapor composition non-uniformity in the reactor, and, eventually, in a non-uniform instantaneous growth rate distribution over the wafer. Because of the non-uniform vapor composition, the GaN growth proceeds under modulated-flow conditions on the wafer periphery and at a nearly constant V/III ratio at the center of the wafer. This may result in variation of the materials properties both in depth and across the wafer.